The invention relates to a device making it possible to measure the reflection coefficient of a structure such as a non-metallic, monolithic or composite structure, when the latter is exposed to microwave radiation.
Such a measuring device can in particular be used for checking the radio transparency of a structure such as that of a radome protecting a radar carried on an aircraft. Other applications of said device can be envisaged without passing outside the scope of the invention.
Radomes protecting radars equipping aircraft can be subject, both on the ground and in flight, to impacts liable to damage them. Thus, they can be struck by an engine when on the ground. They can also be struck by birds or lightning when the aircraft is flying.
Damaged radomes undergo repairs in order to restore their integrity. However, even though such repairs give the radomes their mechanical integrity, they sometimes lead to a modification of the transparency to radiation emitted by the radar in the repaired area. This modification can lead to an error concerning the position and/or distance from an obstacle or in the extreme case the non-detection of an obstacle.
It is therefore desirable to have a measuring device making it possible to control the radiotransparency of radomes after the latter have undergone repairs. Such a device could also be used for controlling the radiotransparency of new radomes during their manufacture.
In order to carry out such measurements, it is known to use an anechoic chamber into which the radome is introduced. However, this is a cumbersome and costly procedure, which does not make it possible to carry out the control or inspection in the workshop or on site.
Measuring devices used in the laboratory are also known, which are called vector network analyzers, which make it possible to simultaneously determine the amplitude and phase of the reflection coefficient of a structure exposed to microwave radiation. These devices, which operate in a wide frequency range, are heavy (15 to 20 kg) and cumbersome. They are therefore relatively unsuitable for use in the workshop or on site.
In addition, due to the large overall dimensions and weight of such a device, the microwave applicator used for emitting the microwave radiation to the structure to be controlled and for receiving the waves reflected by said structure, must be separate from the remainder of the device and connected thereto by a microwave lead which is as flexible as possible and which has an adequate length to make it possible to displace the microwave applicator over the entire surface of the structure to be controlled. However, said microwave lead is a source of problems, because it has at its ends fragile connectors and any twisting of said leads to distortions to the phase of the microwave signals which it transmits. Therefore the measurements performed can be subject to sometimes significant errors with respect to the phase of the signal.
U.S. Pat. No. 3,025,463 to Luomaetal describes a device making it possible to measure the amplitude and phase of the reflection coefficient of a structure to be controlled.
In the first embodiment described, it is a portable device incorporating a single high frequency generator directly connected by a waveguide to a high frequency applicator. In the waveguide, part of the incident wave and part of the reflected wave are sampled in order to be passed to a mixer, which calculates the difference thereof. Manual regulating systems make it possible to modify the amplitude of the fraction of the incident wave directed to the mixer and the phase of the fraction of the reflected wave directed to said same mixer. These settings are carried out so as to cancel out the signal received by the detector. The corresponding values then represent the amplitude and phase of the reflection coefficient of the structure. Although this device is completely portable, its utilization is particularly long and clearly inappropriate for industrial control and inspection purposes.
In the second embodiment described in the Luoma reference, the measuring device is equipped with an additional circuit making it possible to automatically cancel out the signal received by the detector. However, it is a much heavier and more voluminous device, which has a fixed part and a mobile part interconnected by a cable. This cable carries both electric signals and microwave radiations, leading to the disadvantages due to the presence of a flexible, relatively long microwave lead already referred to in the case of vector network analyzers.